This is the only currently referenced site that has the Everyready high voltage batteries http://www.newark.com/find/CatalogBrowseMfg.jsp number 489 a 225v at $94 each and two connected in series would be 450v or the number 497 a 510v at $98 droped down thur a resistor could give 450v.

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Porbably the best option is to keep an eye on online auctions or old camera stores to find one of the RG base units for your Strobomatic 500. The RG base replaces the HV battery with rechargable NICads. These units can be easily refurbished with new NICads.
Often the RG base can be had for a lot less than the cost of the HV battery (if one can be found). You may also want to look for an AC base unit that plugs into the wall for indoor shots....